The present invention relates to a method and apparatus for producing turbine single-crystal gas turbine engine components having integral sealing structures.
Gas turbine engines include a number of blades and vanes that interact with fluids moving through these engines. There is a need to provide seals that limit fluid flow out of designated areas, and such sealing is particularly important in a hot section of a gas turbine engine where the escape of hot fluids can damage engine components and cause reduced engine performance. In the past, seals such a stand-alone labyrinth seal disks have been attached to rotor disks carrying turbine blades, using bolts or other means. These labyrinth seal disks can provide air seals between the rotating structures associated with the blades and adjacent non-rotating structures, which typically include a non-rotating abradable seal structure positioned in close proximity to outwardly extending portions of the rotating labyrinth seal disk. More recently, it has been desired to provide labyrinth sealing structures such as knife edges and hammerhead structures that extend directly from a root portion of turbine blades. In other words, rather than providing separate seal structures, the seal structures are integrated directly into the blade. Examples of such integrated seal structures are disclosed in published United States patent applications US2006/0275106A1, entitled BLADE NECK FLUID SEAL; US2006/0275107A1, entitled COMBINED BLADE ATTACHMENT AND DISK LUG FLUID SEAL; US2006/0275108A1, entitled HAMMERHEAD FLUID SEAL; and US2007/0098545A1, entitled INTEGRATED BLADED FLUID SEAL, each being assigned to the Assignee of the present application.
The manufacture of labyrinth sealing structures such as knife edges and hammerheads that extend outward from root portions of turbine blades presents a number of problems. The blade can be cast in a rough, relatively imprecise manner and then machined to desired shapes and dimensions with precision, forming the labyrinth sealing structures solely by way of the machining process. However, it is generally desirable to reduce machining work as much as possible to make manufacture easier and to reduce material costs. Moreover, the configuration of typical turbine blades can make machining of labyrinth sealing structures difficult or impossible, or can undesirably limit design options for the labyrinth sealing structures. For example, a platform of a turbine blade can extend outward in such a way that machining tools cannot be maneuvered to form desired labyrinth sealing structures located in close proximity to the platform, due to obstruction by the platform.
Furthermore, conventional casting methods would generally not produce as-cast structures with sufficient dimension and true position tolerances for labyrinth sealing structures such as knife edges and hammerhead structures, which must align with adjacent structures in order to properly function in an engine. Conventional casting is also problematic because blades having single-crystal grain structures, as are commonly used in hot sections of modern gas turbine engines, will not allow the growth of the single crystal grain structure through relatively complex, long and slender structures like knife edges and hammerhead structures. Also, turbine blades are normally cast in a tip-down orientation, and in that the orientation of labyrinth sealing structures generally requires grain growth in a sideways or downward direction during solidification of the casting material, which is not feasible for single-crystal growth.